Al Gore's fledgling presidential campaign is being dogged by the allegation that he sided with US pharmaceutical conglomerates to deny South Africa access to affordable Aids medicine.

A group of protesters from Act Up, a pressure group for Aids sufferers, have followed the vice-president around the country. On Sunday they interrupted a campaign speech at a New Hampshire fundraising picnic, chanting: "Gore's greed kills."

They were removed, but not arrested, and the vice-president promised to discuss the issue with them.

But the dispute has refused to die down, and it is beginning to tarnish his image with his core liberal supporters.

The controversy centres on an act which the South African parliament passed in 1997, allowing local companies to produce cheap generic forms of the expensive drug-cocktails used to keep Aids under control.

It provided for the government to pay a fixed "wholesale" fee to the patent holder, potentially reducing the price of the drugs by up to 90%. The medicines and related substances act allows "parallel importing" - the purchase of brand-name drugs from wherever in the world they are cheapest rather than directly from the patent-holder.

Pretoria argued that otherwise it could not hope to treat the 3m South Africans - 8 per cent of the population - with the HIV virus, which causes Aids.

The US pharmaceutical companies which hold the patents for many of the drugs challenged the law in the South African courts on the grounds that it infringed intellectual property rights. They also called on Washington to fight for their interests.

Mr Gore took a leading role in negotiations with President Thabo Mbeki. According to a state department report last February, he played a key part in an "assiduous, concerted campaign" to persuade the government of South Africa to change the law.

The main drug manufacturers lobby, the Pharmaceutical Research and Manufacturers of America (Phrma), is a contributor to the Gore campaign and one of its lobbyists is Anthony Podesta, the brother of the White House chief of staff John Podesta - a friend and adviser to Mr Gore.

James Love, director of the Consumer Project on Technology, said: "To me, Gore has never in his life bucked corporate interest. He hasn't shown any character by bucking big money. It makes you worried he never will."

But the vice-president has been backed by the Aids Action Council, the largest US Aids group.

Its director, Daniel Zingale, said that the anti-Gore protests were misdirected. "Like blaming Roosevelt for the Holocaust."

South Africa was placed on a US watchlist of countries suspected of infringing international patent law, implying the threat of economic sanctions if its act, currently suspended pending a ruling by the supreme court, enters into law.

Aids activists in South Africa say that local firms can manufacture drugs at a small fraction of the cost of buying them from western pharmaceutical giants, which they accuse of charging exorbitant prices. A British firm, Glaxo Wellcome, has been targeted this year by protesters demanding that it make its Aids drug AZT available at cost price to pregnant women with HIV.

But the activists argue that South African companies could produce the drug for a small fraction of Glaxo Wellcome's price and still make a reasonable profit.

In a letter in June to the chairman of the congressional black caucus, Mr Gore said that he supported South Africa's attempt to find a cheap treatment for Aids as long as it did not violate international laws protecting patents. Mr Gore agrees with the drug companies that the South African act does violate those laws.

But South Africa's supporters point out that the World Trade Organisation regulations on intellectual property rights allow patent laws to be waived in the case of a national emergency, cases of extreme urgency, or where there are no profit motives.

The South Africa Aids crisis, they believe, meets all those criteria. South Africa has the fastest growing epidemic in the world, with an estimated 50,000 new HIV cases a month.